1. Field of the Invention
Example embodiments of the present invention relate to a phase-shift mask (PSM) and a method of forming the same. More particularly, example embodiments of the present invention relate to an attenuated PSM for minimizing an intensity difference between a 0th order beam and a 1st order beam, and a method of forming the same.
2. Description of the Related Art
As semiconductor devices continue to become more highly integrated, design rules for the devices are becoming gradually reduced so that the critical dimensions (CDs) of the semiconductor devices are currently becoming scaled down to about 0.07 μm, or less. The above reduction of the design rules and the CDs necessarily causes the various patterns for the semiconductor devices to have high resolution.
Various resolution enhancement technologies (RETs) have been applied to processes for manufacturing semiconductor devices so as to form high-resolution patterns. For example, methods have been suggested for increasing the numerical aperture (NA) of a lens so as to irradiate illumination light onto a minute area of an object in an exposure system, and improving the illumination light to have a short wavelength through a dipole or a cross-pole illumination process. Particularly, a krypton fluoride (KrF) excimer laser having a wavelength of about 248 nm, an argon fluoride (ArF) excimer laser having a wavelength of about 193 nm and a fluorine (F2) excimer laser having a wavelength of about 157 nm are widely used as the illumination light of the exposure system in accordance with the technical trend of high-integration degrees of semiconductor devices. Accordingly, the resolution of a pattern may be sufficiently increased by using short-wavelength light as the illumination light. However, the short wavelength light also causes deterioration of depth of focus (DOF) in an exposure process. For that reason, the RETs commonly adopt a phase-shift mask (PSM) so as to avoid deterioration of the DOF. An initial PSM includes various stepped portions that are formed or arranged in a transparent substrate, and thus the phase of the light penetrating through the PSM is shifted by the stepped portions. However, more recent PSMs have been configured to include an additional layer that is formed on a transparent substrate, and thus the phase of the illumination light is shifted by the additional layer. Particularly, an attenuated PSM has been widely used for forming a large-aspect-ratio pattern such as a contact hole, or an isolation pattern.
The attenuated PSM may shift the phase of the illumination light and control the transmittance of the illumination light using a single layer or a double layer in such a manner that the intensity of a 0th order beam becomes similar to that of a 1st order beam of the illumination light. As a result, the attenuated PSM allows an object to undergo a uniform exposure in an exposure system. A 2nd order or higher beam of the illumination light may hardly be irradiated onto the same position as the 0th order beam due to the recent reduction in pattern sizes. For that reason, the light intensity of an illumination site on the object is generally estimated based on the 0th and 1st order beams of the illumination light. That is, when an intensity difference (hereinafter referred to as intensity deviation) between the 0th and the 1st order beams is within an allowable range, the 0th and the 1st order beams irradiated onto an exposure site of the object may be substantially treated as a single beam having a uniform intensity, and thus a circuit pattern on a mask may be accurately transcribed onto the object.
However, the recent reduction of CDs and pattern sizes of semiconductor devices may also cause a decrease of the transmission area of the attenuated PSM, to thereby increase the intensity deviation between the 0th and the 1st order beams. As a result, the solubility of a first portion of the exposure site onto which the 0th order beam is irradiated can be different from that of a second portion of the exposure site onto which the 1st order beam is irradiated, and thus there is a problem in that the circuit pattern on the mask may not be accurately transcribed onto the object.
FIG. 1 is a graph showing intensities of the 0th and the 1st order beams diffracted by a conventional attenuated PSM. In FIG. 1, the vertical axis represents beam intensity, and the horizontal axis represents a pattern size.
Referring to FIG. 1, as the pattern size becomes smaller, the intensity deviation between the 0th and the 1st order beams becomes greater. Particularly, as the pattern size decreases, the intensity of the 0th order beam is decreased and the intensity of the 1st order beam is not substantially changed. As a result, as the pattern size decreases, the intensity deviation is increased. Particularly, the intensity deviation when the pattern size is about 40 nm is about two times the intensity deviation when the pattern size is about 100 nm.
In an effort to decrease the intensity deviation between the 0th and the 1st order beams, there has been suggested that a phase-edge PSM (PEPSM), which compensates for a phase shift at an edge of a light-shielding pattern, or a chromeless mask (CLM), be used in place of the attenuated PSM. However, there is a problem in that use of the above PEPSM or CLM requires an additional process, which can decrease process efficiency in a manufacturing process of a semiconductor device.